1. Field of the Invention
This invention is related to nuclear magnetic resonance (NMR) techniques and, more specifically, to shimming the magnet of an NMR system to compensate for inhomogeneities in the field of the magnet.
2. Description of Related Art
A homogeneous magnetic field is critical for NMR experiments. Imaging experiments such as gradient echo-based techniques, localized spectroscopy and echo-planar imaging are a few experiments that can fail if the homogeneity of the magnetic field within a sample being tested is poor. While spin-echo imaging is more tolerant of inhomogeneities, a homogeneous magnet will nonetheless give better results.
In conventional NMR systems, the field of the magnet is "shimmed" by locating "shim-field" coils at various positions relative to the field of the magnet. As appropriate currents are passed through the shim field coils, they produce characteristic fields which compensate for the inhomogeneities of the magnet. However, achieving the appropriate characteristic fields in the shim coils often requires adjusting the current through the coils manually. This procedure can be tedious and time-consuming. In high-resolution systems, it is not unusual to shim a magnet for hours, or even days, to achieve a certain homogeneity specification. This procedure can be automated by employing a simplex procedure, but that too can be time consuming, and may not always give the optimum results.
A more analytical solution to the problem is to determine the inhomogeneous field distribution of the magnet, and calculate the currents of the shim coils necessary to compensate for the inhomogeneities. The field of the magnet can be measured by using a small NMR probe which is moved through the magnetic field by a special mechanical device. However, this method is also time-consuming. Furthermore, the probe itself causes a distortion in the field, and the measurement does not correspond to the actual field that would influence a sample being tested.
A number of automated shimming procedures have been reported which make use of NMR imaging. These methods are beneficial because: 1) the field measurements are done while a sample of interest is present in the field of the magnet, and the field inhomogeneity seen by the sample is directly measured and compensated for; 2) knowing the field or frequency distribution within the sample allows the specification of an arbitrary shimming region of interest; 3) shim field maps are calibrated using the same measurement methods and, therefore, and misregistration in the image caused by misalignments, non-linearities and imperfections in the gradient or shim coils are self-compensated; and 4) the shimming procedures can be fully automated.
Three-dimensional (3D), two-dimensional (2D) and one-dimensional (1D) image-based autoshimming methods have been demonstrated in the past. The field-mapping techniques used involve imaging pulse sequences and therefore require special three-axis field gradient hardware for performing the imaging experiments in high resolution NMR spectrometers. The field mapping experiments are specially designed to measure small residual field inhomogeneities in the magnet.
One disadvantage of the field-mapping techniques of the past is that pulsed field gradients can induce eddy currents in the metal components of the magnet which result in additional, time-varying (eddy current) fields. Unless the residual fields caused by the eddy currents are kept to a negligible level, the shimming procedure will produce erroneous results. Shielded gradient coils have been used in the past which are specially designed to minimize eddy current effects by restricting the stray field to within the bore of the coils. However, in high-resolution applications, non-negligible eddy currents may be generated even when using shielded coils.
The actively-shielded gradient coils currently in existence for use with modern gradient-enhanced spectroscopy experiments are ideally suited for field mapping experiments. However, the current design of these gradient coils are such that they are an integral part of a particular radio frequency (RF) probe, which limits their use for shimming. Limited space in many existing narrow-bore, high-resolution magnets makes it impractical to design a single, universal gradient set that can be used with the various RF probes that are used in a spectrometer. Furthermore, installing gradients on each RF probe may not be a practical solution because of cost considerations.
All NMR spectrometers are equipped with a set of linear gradient coils (unshielded) positioned along the x, y and z axes, respectively. However, these coils are not suitable for existing imaging experiments involving pulsed field gradients, mainly because of the slow response (rise and fall) times of the coils, and the eddy currents which would be induced during the process.